Conventionally, for viewing of digital video contents or recording of digital data, various types of optical recording media have been widely employed, including CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD±RWs, and DVD-RAMs. On the other hand, these types of optical recording media have been demanded of ever-increasing recording capacities. To meet these requirements, a start has already been made at preparing for the market so-called next-generation DVDs as products that can store massive video images and data. The next-generation DVD has an increased recording capacity which has been achieved by reducing the wavelength of the laser beam, used for recording and reading, to as short as 405 nm.
For example, the Blu-ray Disc (BD) standards or one of the next-generation DVD standards employs a setting of 0.85 for the numerical aperture of the objective lens, thereby enabling recording and reading of 25 GB on a single recording layer.
However, further increases will be expected in the capacity for video images and data in the future. In this context, a technological breakthrough concerning a so-called multi-layered optical recording medium has been suggested to provide it with an increased number of recording surfaces to thereby increase its capacity. Another technology has also been suggested for BD-standard multi-layered optical recording media in Non-Patent Document 1 by 1. Ichimura et. al., Appl. Opt., 45, 1794-1803 (2006) or in Non-Patent Document by K. Mishima et. al., Proc. of SPIE, 6282, 62820I (2006). Those recording media are provided with 6- to 8-layer recording surfaces, thereby realizing as very large a capacity as 200 GB.